The present invention is directed in general to a fastener drive system, and more particularly to a new and improved fastener and driver assembly wherein the fastener includes a drive socket and the driver includes a tapered bit of novel design.
Tapered drive bits for transferring rotational torque to correspondingly structured fasteners are well known. Such arrangements find considerable application, for example, in high volume mass production assembly lines wherein a power tool having a tapered bit is used to drive fasteners into a secured position. The tapered nature of the bit enables the fastener to be engaged thereon with a slight friction fit such that the fastener will remain in mounted relation on the end of the bit until it can be engaged and driven.
Prior art drive systems i.e. socketed fastener and drive bit, or socket driven and external fasteners driving head, have taken a wide varitey of forms, two of the more common being the "hex" head and "Phillips" head types. Most systems rely upon the use of components of similar mating shape with the mating portion thereon being defined by reltively planar surfaces. While these systems have proven satisfactory for many applications, in recent years multi-lobular type drive systems have been developed, which have proved superior for use in applications involving controlled high seating torques. In this regard, the multi-lobular system, unlike many prior art designs is possessed of high efficiency in converting applied force to driving torque, and these multi-lobular systems are also effective in reducing the radial force components which tend to damage the socket element of the systems. One such multi-lobular system is illustrated in applicant's U.S. Pat. No. 3,584,667.
As a further matter, it is relatively common practice with respect to "hex" and "Phillips" type systems to employ tapered bits, in conjunction with a socket head fastener wherein the socket walls are parallel or only slightly tapered, to attain a desired degree of frictional engagement upon mounting of the fastener on the end of the bit. This feature is useful, in that the fastener will remain mounted on the bit during a moment preparatory to proper positioning of the fastener for driving. As an additional matter, use of a tapered bit serves to take up or obviate socket size variances that may be encountered. That is to say when a friction fit is obtained, the fastener is firmly engaged with the bit and will not "wobble" during driving. It has been proposed to apply this concept to multi-lobular drive systems for attainment of the same advantages. Unfortunately, however, these attempts have not proven totally successful as certain, unexpected problems have been encountered, as will be explained.
More specifically, with respect to all types of drive systems, the ability thereof to handle applied force, and convert same to driving torque is dependent upon the depth of engagement of the respective male and female components. If insufficient depth of engagement is attained between the driver bit (male component) and the fastener socket wall (female component), the material available by the socket wall to resist the force being applied by the driver bit are structurally insufficient, and the socket wall will deform under the load. The critical nature of this factor will be appreciated more fully when it is also considered that often the drive bits are formed of hardened alloy steel, while the fastener socket wall is formed of much softer varieties of steel, also a consideration is the wide variance in dimensional tolerance that is encountered between fastener and driver due to many factors, such as different manufacturers and wear of the bit and socket forming tooling in service. These tolerance variations often result in interfering engagement between the fastener and the drive occurring before the desired minimum depth of engagement is realized.
The attainment of an assured depth of engagement with multi-lobular drive systems has proven no problem where the surface portions of the mating bit and socket are disposed generally parallel to respective axis. However, where employment of a tapered, multi-lobular bit has been attempted, the results obtained have not been satisfactory, with respect to the high degree of uniform standards demanded by the fastener industry. More specifically, in inserting a tapered bit into a straight or less tapered socket, some measure of interference (i.e. friction of fit) will be attained at a certain depth of engagement. The problem is to uniformly attain this interference after the desired minimum depth of engagement is reached.
With a tapered multi-lobular drive system, the lateral or axially extending portions of the bit and socket are much increased over other designs; as for example a "hex" system. Correspondingly, any variance in dimensional tolerances which might be encountered, increases the possiblity that surface-to-surface interference or frictional engagement will occur, before the discussed depth of engagement is attained. The present invention provides a novel solution to this problem by enabling the attainment of the advantage of a tapered bit, i.e. vertical mounting of the fastener or the bit, with a greater degree of assurance of attaining the minimum depth of engagement between the driver bit and the socket.
It is therefore a general object of the present invention to provide a new and improved fastener drive system, wherein the fastener includes a drive socket, and wherein the driver, although tapered is structured for entering a fastener drive socket to a consistent and predetermined depth of engagement to assure efficient torque transfer from the driver to the fastener. The above mentioned object is achieved by providing a fastener drive system comprising, a fastener member having a drive socket, wherein said socket is multi-lobular in cross section and has an inner surface formed by alternating and smoothly joined flutes and lobes, and a driver for imparting rotational torque to the fastener and including a head portion having a tip end and a center axis. The head portion of the driver is correspondingly multi-lobular in cross-section having an outer surface formed by alternating and smoothly joined flutes and lobes for mating engagement in said fastener socket. More specifically the driver surface portions which define the base of the flutes, which it is recalled mate with the socket lobes are formed generally parallel to the center axis of the driver. On the other hand the surface portions of the bit are tapered axially, converging in a direction towards the tip end. As a result of this design the possibility of dimensional variances producing an undesired interfering engagement prior to attainment of the proper depth of engagement are substantially reduced, if not eliminated. More specifically, since the flute surface portions are substantially parallel to the bit axis attainment of premature interfering engagement along the interfaces with the socket lobe surfaces will not occur. As such the only area of possible interfering engagement is along the tapered external surface portion of the lobes. Since these lobe surface portions are external, the dimensional tolerance thereof can be controlled more readily to attain the desired mode of operation. Further, even if some dimensional variances are encountered, the likelihood of these adversely effecting the performance of the drive systems are materially reduced, since interfering engagement can occur over only a small portion of the entire surface area of the bit.